NORTH CHARLESTON, S.C. (AP) — Researchers say they may have the final clues needed to solve the mystery of the Confederate submarine H.L. Hunley, which never resurfaced after it became the first sub in history to sink an enemy warship, taking its eight-man crew to a watery grave.

Scientists said Monday that the Hunley apparently was less than 20 feet away from the Housatonic when the crew ignited a torpedo that sank the Union blockade ship off South Carolina in 1864. That means it may have been close enough for the crew to be knocked unconscious by the explosion, long enough that they may have died before awakening.

For years, historians thought the Hunley was much farther away and had speculated the crew ran out of air before they were able to return to shore.